The bit rate of high speed telecommunications transmission systems, such as those which use the DS3 or STS protocols, are sufficiently high (e.g. 45 and 51 Megahertz) that less than ohmic couplings of the transmission lines seriously degrade the data. For bit rates of this order of magnitude, transmission line stubs having lengths on the order of inches also have this effect.
Transmission systems that take in DS3 or STS signals in large numbers often employ a 1-for-n protection scheme, whereby n working transmission/receiver interface cards are protected by a single spare interface card. More generally, there exist m-for-n protection schemes wherein m spare interface cards protect n interface cards. A spare card is used in the event of a failure of one of the n interface cards.
Incoming and outgoing signals can be routed to the spare card in a variety of ways. According to one conventional system, the signals are passed first to a switch card which can then either route the signals to the working interface cards or to the spare card. The problem with this approach is that if the switch card is removed from the system, the paths to both the working and spare cards are broken, and the signals are lost. Some systems attempt to employ special connectors which, when the card is removed, connect the signal paths. Unfortunately, such connectors have proven to be unreliable in service.
In an attempt to solve this problem, signal splitters have been used on the input side to receive the incoming signal and pass the signals from the splitters to both the working interface cards and the switch card. With this scheme, the switch card can be removed without losing the signal to the working card. The problem then becomes the failure of the signal path when the splitter card fails. Another approach to this problem is to distribute the function of the switch card among the working interface cards. This can be complex, may again require unreliable shorting connectors, and may pose difficulties in maintaining the integrity of high-speed signals when they pass through several connectors. These conventional methods also require a large number of switching events or "switches". Purchasers o f high-speed telecommunication systems judge the reliability of these systems by the number of switches which are made. A need therefore exists to develop a spare card protection scheme which minimizes the number of switching events and better maintains the signal path and the data of the signal.